How do I make a VPN tunnel with no encryption?

I have a vpn tunnel between a 3640 and a pix 520. We do a lot of video conferencing and the 3640 processor maxes out because of the encryption. I want to keep the tunnel, but not the encryption. What lines do I need to change if this is possible. We have 10. networks at both ends, that's why I need the tunnel. Thanks in advance.

This is actually a pretty cool feature, i didn't even know it existed until I was looking for a solution to advertise a subnet (prefix in BGP talk), only if a certain condition existed. This is exactly what conditional advertisements does
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Attached policy provides CLI access to the Cisco 4G router over text messaging. Two files are in the attached .tar file:
1. commandoversms.tcl
2. PDF with instructions on how to load and use the .tcl file.